Using short term cultures of peritoneal macrophages from the mouse, we have developed a system in which we can assess the release of colony stimulating activity (CSA) by these cells. We have found that there is a factor in serum which is necessary for this release. This project proposes to evaluate and characterize this factor. Using a line of thymus epithelial cells (TEPI) developed in this laboratory and which have been shown to be potent sources of CSA, several studies will be performed. The effect of serum and other factors on CSA release by these cells will be studied. Thymocyte and bone marrow cell interaction with the epithelial cells and conditioned medium from cultures of TEPI cells will be studied using liquid and semi-solid medium. Hemopoietic stem cells which we have shown to be present and normal adult mouse liver will be studied to determine if they have characteristics of their fetal or adult bone marrow counterparts. BIBLIOGRAPHIC REFERENCES: Hays, E.F., Craddock, C.G., Haskett, D. and Newell, M.: In Vitro Colony Forming Cells in the Marrow of Leukemic & Preleukemic Mice. Blood 47:603-610, 1976. Firkin, F.C., Hays, E.F., and Cline, M.J.: Effect of Hypertransfusion on Granulopoiesis in Bone Marrow Depression: Studies in the Irradiated Mouse. British J. of Haematology, In Press, 1977.